The present invention relates generally to pneumatic valves and, more particularly, to a directly piloted valve assembly.
Generally speaking there are two main valve types, direct acting designs and piloted designs. With a direct acting solenoid design the movable armature is in direct contact with and directly pushes or pulls the main valve shifting element, typically called the spool or stem. After shifting, the spool or stem is returned to an original position by the force of a return spring or, in the case of a double solenoid valve, by a second solenoid. Sealing elements on the stem are typically of the poppet configuration. Poppet seats allow for a shorter stroke of the spool for a given amount of flow gap between the poppet and seat. Poppet design consists of elastomeric sealing material, the poppet, formed about the circumference of a movable spool. The poppet makes a seal when it is forced against a relatively sharp edge within the valve body.
One significant limitation of the direct acting poppet design is the requirement of a short spool stroke, and the tight tolerances involved in manufacturing a valve body and spool to accommodate a short spool stroke. The short spool stroke is needed in order to keep solenoid power as low as possible. Unfortunately, the short spool stroke increases the difficulty in the manufacturing process. Each valve is assembled with its own unique positioning of poppets on the spool. This requirement is most demanding for the 4-way valve because two (2) poppets must seal at the same time. Sealing both poppets at the same time is made more difficult because one poppet seals with pressure and the other seals against pressure. One poppet is forced against the sealing diameter by pressure and the other is being forced away from the sealing diameter. In addition to the tight positional tolerances of mating components, the seats in the body must be precisely machined and aligned. The seat finish has to be smooth, with no dings dents or rough machine marks, and the radius of the seat has to be within a tight tolerance, typically around 0.007/0.003 inches. If the seat is too large, the valve will not seal at high pressure and if the seat is too small (sharp), the poppet will be cut, causing an early failure.
In addition to the manufacturing issues, the need for a low-friction, short spool stroke essentially prevents the use of a sliding seal on the spool. A sliding seal would require a much longer stroke of the spool than a poppet seal, because the sealing element in a sliding seal design has to move far enough to create the flow gap in addition to the distance moved to exit and enter the sealing bore. Sliding seals that enter and exit a bore are typically not used with the direct acting design because the long stroke and high seal friction forces would require high solenoid power levels. There are designs employing close metal to metal fits to achieve the sealing function, this results in low friction forces. The negative aspect of this type of design is that there is always some leaking. The sliding metal to metal elements must have a clearance and so the always present clearance gap has an always present leak.
Despite the noted disadvantages, there are a number of advantages to direct acting designs, including: (1) there is no minimum operating pressure limitation; (2) typically, these designs will function with both vacuum and pressure; and (3) the total part count is low (i.e. less than the piloted design described below). Disadvantages of the direct acting design include: (1) relatively low ratio of flow to solenoid power (i.e., a relatively high wattage input is required for the solenoid to control a relatively low flow capacity); (2) the relatively high cost of manufacturing parts to tight tolerances or making assembly settings to tight positional tolerances; (3) for the custom assembled version, parts can not be changed or replaced (product is not repairable and typically the valve coil can not be changed when a different voltage is needed); and (4) for products machined to tight tolerances only, not needing custom assembly, coils can be changed but the spool cannot be changed because insertion of the spool requires special tooling.
The typical piloted valve is really two valves, combining a larger high flow 4-way or 3-way valve body with a smaller 3-way or 4-way valve that is typically attached at one or both ends of the larger body. The smaller valve provides an air pilot signal that acts on a pilot piston of the larger valve to shift a spool. Return of the spool can be accomplished with either air pressure or a return spring or a combination of both. This design results in relatively high flow with lower power consumption than a direct acting valve. Some advantages of piloted design include: (1) the possibility of greater flow through the valve than the direct acting design because the spool stroke is not limited by the power of the solenoid coil; (2) the valve components can be replaced to repair the valve or change the coil voltage; (3) the valve does not require tolerances as tight as those of the direct acting design.
Some disadvantages of piloted design include: (1) the part count is typically higher than in direct acting designs because both a pilot valve and a main valve are required; (2) the final product is larger due to the extra components, and may not be useful in applications requiring a small valve; (3) there is a minimum pilot pressure required for pilot function, around 30 pounds per square inch; (4) piloted designs typically have a slower response time than direct acting designs; (5) piloted designs typically have a more expensive valve body due to requirement of connecting the pilot valve output to the pilot piston and the supply pressure to the return piston. This is most commonly done with cross drillings in the valve body that are plugged with balls or synthetic elastomer seals; (6) piloted designs cannot be used with a vacuum unless a separate pilot pressure is supplied; (7) different valve bodies must be machined for different pilot port sources; (8) the power of the 3-way pilot valve is determined by orifice size of the pilot valve. Pilot valves have very low Cv (flow capacity) values because the forces of the solenoid armature and the return spring have to be greater than the product of the orifice size and pressure (area X pressure), this is because the typical 3-way pilot valve is of the unbalanced poppet design therefore higher pressures require higher forces from the armature. The small orifice size allows for lower coil power but the trade off is slower main valve response due to reduced flow to and exhaust from pilot pistons; (9) the spool requires 6 seals to prevent extrusion (i.e., deformation of the seal into the hole that is being sealed).